The invention relates to low pressure fluorescent discharge lamps, and more particularly, to a method for improving long-term operating appearance of a low pressure discharge lamp which utilizes a mixture of a homogeneous three component phosphor blend carried as a layer on the interior surface of the lamp envelope. The phosphor blend substantially comprises a blue-violet-emitting phosphor component, a red-orange-emitting phosphor component, and a green-emitting phosphor component which is zinc silicate activated with manganese.
In U.S. Pat. No. 3,858,082 dated Dec. 31, 1974, issued to William A. Thornton, Jr. there is disclosed a fluorescent lamp that generates three narrow emissions in specific blue, green and red regions of the spectrum. This lamp generates composite radiations of a "white" color very efficiently and with excellent color rendition of illumnated objects. The overall performance of these lamps is excellent and they have received wide acceptance in applications such as department and specialty stores, an example being fabric stores, where the need for excellent color rendition of illuminated objects is apparent. In some limited cases, however, the "green emitting" phosphor component of the blend has displayed a somewhat poorer so-called maintenance of light emission than the other phosphor components at locations proximate the ends of the envelope appearing "pink" as compared to the rest of the operating lamp, as viewed directly. While this has not impaired the overall lamp performance as a source of illumination for objects, some users object to any pink appearance of the end portions of the envelope of the operating lamp.
Application of a phosphor coating to the interior surface of a fluorescent lamp envelope is well known in the art. For example, U.S. Pat. No. 3,303,042 dated July 2, 1962 issued to Reed et al., discloses a method of applying a phosphor coating of uniform thickness to the interior surfaces of a fluorescent lamp envelope of irregular crosssectional shape by means of an aqueous suspension phosphor and a water soluble polymer.
Washing phosphors is known in the prior art as a method for improving lamp maintenance. For example, in U.S. Pat. No. 3,757,938 issued to A. Wachtel dated Sept. 11, 1973, there is disclosed a method of sizing a zinc-magnesium silico-germanate phosphor to remove the ultra-fine particles to improve the performance of a discharge lamp comprising suspending the finely-divided prepared phosphor in an aqueous solution of ionizable aluminum compound or diethylene triamine pentacetic acid chelating agent, then settling the phosphor and removing the ultrafine particles which remain suspended. Another example is given in U.S. Pat. No. 3,165,476, issued to A. Wachtel, dated Jan. 12, 1965, which discloses a method for processing fired, finely-divided, copper-activated Group IIB metal sulfide electroluminescent phosphor in order to improve the brightness and efficiency of the phosphor. The method entails washing the phosphor in a solution which is a good solvent for Group IIB metal sulfide for a sufficient period of time to lighten the body color of said phosphor, additionally washing the phosphor in a 5% to 15% by weight hydrogen peroxide aqueous solution for a period of at least 30 minutes and not exceeding such time as will permit a substantial portion of the phosphor to dissolve, and thereafter removing from the phosphor any residual hydrogen peroxide washing solution.